Southern right whales were decimated in the 19th century in New Zealand waters.

Brian Skerry/National Geographic Photography Fellow

The population of southern right whales in the waters off New Zealand is just 12 per cent of its size before whaling began, according to a new study.

Key points whales

Key points

Between 28,000 - 47,100 southern right whales existed in NZ waters before whaling

Numbers dropped in early 20th century to 15 - 20 mature females

By 2009, there were an estimated 2,000 southern right whales in NZ waters

The research, published in Royal Society Open Science, highlighted the slow path to recovery from whaling in this area, said the study's lead author, Jennifer Jackson from the British Antarctic Survey.

"It's really easy for us to forget how different our oceans looked before we went in and exploited them," Dr Jackson said.

"There are anecdotes that people in Wellington would complain about the noises that the southern right whales were making in the harbour at night."

In the 19th and 20th centuries, southern right whales (Eubalaena australis) were massacred by whalers in their coastal calving grounds around the New Zealand mainland and while foraging in the waters around New Zealand and south-eastern Australia.

Even after the southern right whale was protected in 1935, secret and illegal Russian whaling in the 1960s took the surviving populations in New Zealand waters to the brink of extinction.

Dr Jackson said southern right whales were particularly vulnerable to whaling because they swim on the surface, are slow, and return faithfully to the same shallow waters to breed in a three-yearly cycle.

Genetic and historic data build a picture of whaling

To understand how fast the population declined — and how fast it is recovering in this area — the researchers combined genetic information from the current population of whales with information obtained through thousands of old whaling log books, and import and export records from New Zealand fisheries for whale products such as oil.

Major current breeding areas are nearshore off southern Australia, New Zealand (particularly Auckland Islands and Campbell Islands), Atlantic coast of South America (Argentina and Brazil), and southern Africa (mainly South Africa).

Known as Tohora by the Maori people.

The analysis indicated that 28,800–47,100 southern right whales occupied waters around New Zealand before whaling began.

However, at the peak of whaling in the early 1900s, that population dropped to as few as 15–20 mature females, before recovering to about 2,000 individuals in 2009.

But, while southern right whale populations are still just a fraction of their original size, they are recovering, said study co-author Dr C. Scott Baker, the director of the Marine Mammal Institute at Oregon State University.

"Right whales seem to be reproducing at rates that are considerably higher than we would have imagined a decade or two ago, based on what we knew about their life history," Dr Baker said.

He said this recovery was due to recolonising of New Zealand's waters by whales from further south.

"What we know is that a remnant population seemed to have survived in the sub-Antarctica Auckland Islands, which are part of New Zealand," Dr Baker said.

Method could be used to study Australian whales

While the research focused on New Zealand's population of southern right whales, the researchers said populations in south-east and southern Australia experienced similar levels of exploitation.

Dr Baker said that while little was understood about the population structure of the southern right whale, there was the possibility that the New Zealand whales may have foraged off Australia's south-eastern coast, and vice versa.

The two populations are genetically very similar, but researchers focused on the New Zealand population because whaling records for the region were reasonably comprehensive.

Dr Jackson said she hoped to apply the same method to examine pre-whaling populations of southern right whales around the east Australian coast.

Claire Charlton, a PhD candidate at the Curtin University Centre for Marine Science and Technology, said the use of such multi-levelled science to get more accurate pre-whaling abundance data was extremely valuable in assessing the recovery of great whales around the world.

Ms Charlton, who is involved with an annual survey of southern right whales off Australia's west coast now in its 25th year, says the western population appears to be recovering better than the eastern population.